This blog provides updated forecasts and comments on current weather or other topics

Wednesday, April 22, 2009

Shiptracks!

During the past several days there have been some odd lines apparent in the visible satellite photos off our coast (see examples). These lines are often seen off the U.S. west coast in thin stratus and stratocumulus cloud decks. Sometimes they look like someone is playing tic-tac-toe over the Pacific. What causes they strange lines? And why was the U.S. Navy concerned about them?

The answer...these are called shiptracks and they result from the particles produced by the combustion in ship engines . But why would smoke and engine effluent cause white lines? Nearly all low level clouds (such as stratus and stratocumulus) are composed of small water droplets that form on small particles, called condensation nuclei. Over the ocean, where air is clean, there are relatively few of such particles--thus there are relatively few cloud droplets...that grow big and fat since they don't have much competition. But if you add lots of particles from the smokestacks of ships the situation changes greatly...now there are many cloud condensation nuclei and you get a very large number of small drops in the clouds.

It turns out that if you have a certain amount of liquid water in a cloud, the reflective properties of a cloud with lots of small droplets is very different than one with a fewer number of large droplets. Specifically, clouds with more small droplets are more reflective and thus appear whiter from space. The bottom line: the particles from smokestacks cause more reflective clouds. So as the ship moves it leaves a line of whiter, more reflective clouds...the shiptracks you see! These shiptracks move with the wind and eventually get distorted and faded.

Interestingly, with many ships moving across the world's oceans, this effect can add up...even neutralizing a small bit of global warming. And the Navy was quite concerned about shiptracks during the cold war, since enemies could see where U.S. ships were by following the tracks. Fortunately, nuclear ships have no such problem..they have no impact on the clouds.

All...contrails are somewhat different. They occur because jet engines produce water vapor which then condenses in the cold air aloft. Water vapor addition is not critical for shiptracks...the condensation nuclei are what is missing..I think I should add shiptracks to the next edition of my book....cliff